The present invention relates to an electric device, especially, to an electric device which has a semiconductor chip mounted on a lead frame and is hermetically packaged.
Integrated semiconductor circuits are most important electric devices which have been broadly used in a variety of fields. One of the problems from the view point of reliability is the flowing-in of moisture or other impurities into the IC chips embedded in mounting resin. The flowing-in takes place through cracks or gaps which may occur in the molding to form paths from the outside of the molding to the surface of the lead frame. The moisture which reaches the IC surface causes undesirable destructive corrosion of the semiconductor constituting the IC chip and leads to malfunction of the chip operation.
FIG. 1 is an illustration showing how defect are formed in the packaged IC device. The structure comprises an IC semiconductor chip 28 mounted on a base frame 35' by means of an organic adhesive 24, leads 37 with which the IC chip 28 is electrically coupled by means of Au wiring 39 and epoxy molding enclosing the IC chip and lead frames 35 and 37. When the IC chip was dieattached on the base plate 35' at a temperature from 100 to 450.degree. C., gases have been usually discharged from the organic adhesive 24 so that organic films 24' have been formed on the lead frames, especially, on the rear surface of the lead frame. The surface of the frame has been also usually oxidized and formed suboxide films 24, 24' and 32 on the surface. In this structure, moisture tends to gather near the interface between the frame and the molding. When the IC device is arranged on an electrical circuit board, soldering is performed by dipping the IC device in a molten solder at 260.degree. C. for 3 to 10 seconds. The rapid change in temperature often causes cracks in the molding as designated by 33 and 31. Also, the moisture trapped around the frames is evaporated and causes swelling 41' to form a cavity 42 by the force of the vapor pressure resulting in cracks 33'. This swelling is particularly likely since the organic films or the oxide film 32 and 24' weaken the adherence power of the molding to the frame.
In particular, organic films and suboxide films which might be formed on the rear surface of the lead frame in advance of molding inhibit the sturdy connection between the rear surface and the molding material and eventually produce the swelling 41'.